The present invention is a device capable of measuring electrostatic potentials with insignificant current flow into the input of the device. Generally, such devices include a probe or sensor assembly working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential. Thus, the features of the present invention may be used in the printing arts and, more particularly, in an electroreprographic system to control a xerographic process. These electrostatic voltmeters, or ESVs, are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality reprographic output.
Heretofore, it has been recognized that the sensing probe or electrode must be modulated, with respect to the field being measured in order to accurately measure the field. Moreover, two methods of achieving the required modulation of the electrode are known, the first requiring a stationary electrode and a vibrating element, or vane to modulate the field which reaches the electrode, as described by U.S. Pat. No. 3,921,087 to Vosteen (Issued Nov. 18, 1975), or by U.S. Pat. No. 4,149,119 to Buchheit (Issued Apr. 10, 1979). The second approach utilizes a moving electrode, affixed to the end of a vibrating element as disclosed in U.S. Pat. No. 3,852,667 to Williams et al. (Issued Dec. 3, 1974), or alternatively as disclosed in U.S. Pat. No. 5,212,451 to Werner (Issued May 18, 1993).
The ESV may also utilize an "ion probe" similar to that disclosed by Williams in U.S. Pat. No. 4,270,090 (Issued May 26, 1981) to produce a signal representative of the potential on the surface being characterized.
Moreover, numerous approaches are recognized to process the signal produced by the sensing electrode, thereby enabling the measurement of the electrostatic field potential. Some signal processing approaches are illustrated in the following disclosures which may be relevant:
U.S. Pat. No. 3,667,036 PA1 Patentee: Seachman PA1 Issued: May 30, 1972 PA1 U.S. Pat. No. 4,027,240 PA1 Patentee: Meade PA1 Issued: May 31, 1977 PA1 U.S. Pat. No. 4,061,983 PA1 Patentee: Suzuki PA1 Issued: Dec. 6, 1977 PA1 U.S. Pat. No. 4,149,119 PA1 Patentee: Buchheit PA1 Issued: April 10, 1979 PA1 U.S. Pat. No. 4,330,749 PA1 Patentee: Eda et al. PA1 Issued: May 18, 1982 PA1 U.S. Pat. 4,853,639 PA1 Patentee: Vosteen et al. PA1 Issued: Aug. 1, 1989 PA1 U.S. Pat. No. 4,797,620 PA1 Patentee: Williams PA1 Issued: Jan. 10, 1989 PA1 U.S. Pat. No. 4,804,922 PA1 Patentee: Sometani et al. PA1 Issued: Feb. 14, 1989 PA1 U.S. Pat. No. 4,868,907 PA1 Patentee: Folkins PA1 Issued: September 19, 1989 PA1 U.S. Pat. No. 4,878,017 PA1 Patentee: Williams PA1 Issued: Oct. 31, 1989 PA1 U.S. Pat. No. 4,973,910 PA1 Patentee: Wilson PA1 Issued: Nov. 27, 1990 PA1 Japanese Publ. 62-90564 PA1 Inventor: Kumada PA1 Publication Date: Apr. 25, 1987
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 3,667,036 to Seachman discloses electrometer amplifier circuits for measuring the potential of the electrostatic charge formed on an insulating surface. The circuit includes a probe assembly consisting of probe and guard electrodes. The output of the probe electrode is connected to a high impedance circuit which comprises a Metal Oxide Field Effect Transistor (MOS FET) in a source-follower configuration.
U.S. Pat. No. 4,027,240 to Meade discloses a voltmeter used in detecting an electronic voltage signal in an ordinance firing circuit, where the voltmeter is protected by a pair of back-to-back parallel-connected limiting diodes. A first limiter is used to reduce the amplitude of the detected signal, while the second limiter prevents overloading of an indicator.
U.S. Pat. No. 4,061,983 to Suzuki discloses a transistor amplifier including a bipolar transistor supplied with an input signal and a field effect transistor (FET) which is directly connected to an output electrode of the bipolar transistor to amplify the applied signal. A protective circuit senses the load impedance and activates a voltage signal which ultimately results in the lowering of the gate potential on the FET to prevent the FET from damage.
U.S. Pat. No. 4,149,119 to Buchheit teaches an electrostatic voltmeter or electrometer which includes a probe sensor for sensing electrostatic charge present on a test surface. The probe sensor is modulated using a rotating vane or shutter arrangement. The probe is also conditioned to receive both A.C. and D.C. signals which are amplified by a D.C. amplifier, where the A.C. signal from the probe is fed back to the D.C. amplifier to stabilize its output.
U.S. Pat. No. 4,330,749 to Eda et al. teaches an electrometer apparatus for measuring the electrostatic potential on the surface of a photoconductive drum. The apparatus consists of an electrode which is placed near the surface on which the electrostatic potential is to be measured. A potential proportional to the surface potential is induced in the electrode and applied to the input of an amplifier with high input impedance. The amplifier has a MOS FET input stage with a high input impedance and a low bias current.
U.S. Pat. No. 4,853,639 to Vosteen et al. discloses a non-contacting type electrometer apparatus for monitoring the electric potential of a test surface. A sensing integrator is used in conjunction with a pre-amp and a high-gain operational amplifier (opamp) to provide an improved high-frequency response.
U.S. Pat. No. 4,797,620 to Williams discloses a non-contacting electrostatic detector which eliminates the use of high-voltage circuitry in non-space dependent, high-voltage electrostatic monitoring devices. An A.C. voltage, having the same frequency as the modulator frequency, is used to produce a zero net current flow, so that the magnitude and phase of the output signal are proportional to the magnitude and polarity of the electrostatic potential monitored.
U.S. Pat. No. 4,804,922 to Sometani et al. and U.S. Pat. No. 4,868,907 to Folkins both disclose devices which proportionally convert electrostatic voltage into current.
U.S. Pat. No. 4,878,017 to Williams teaches a non-contacting electrostatic voltage follower having a response speed independent of the frequency of modulation of the capacitance or electrostatic field between a detector electrode and the measured surface. The voltage follower is capable of following both static and dynamic characteristics of an external field or potential to be measured.
U.S. Pat. No. 4,973,910 to Wilson teaches an electrostatic analyzer that incorporates a field effect transistor (FET) as a sensor used to convert electrostatic voltage into a proportional current. The sensor is described as a semiconductive device having n-p-n junctions. In operation, a zero-field reference is used to alter the base potential of the sensor, thus forming a zero-field condition. The surface potential difference is then determined as a function of the sensor base voltage, which is directly measured by a voltmeter.
The Kumada publication (JA-62-90564) illustrates a circuit for measuring surface potential, including an independent power source which provides power to an impedance converter circuit which is separated from a signal processing circuit.
The present invention includes an electrostatic voltmeter design that employs high-voltage components provided on a single integrated circuit, thereby reducing the need for discrete components in the fabrication of electrostatic voltmeters. By providing the high-voltage components on a single integrated circuit, the performance and reliability of the components are also improved. Furthermore, the high-voltage integrated circuit components may be utilized in both contacting and non-contacting electrostatic voltmeters.
In accordance with the present invention, there is provided an apparatus for generating a low voltage signal proportional to an electrostatic potential on a surface. The apparatus comprises sensing means for producing an output signal representative of the electrostatic potential on the surface; a high-voltage source adapted to produce a first potential of first polarity; high-voltage level shifting means, powered by said high-voltage source, for generating a first signal referenced to ground in response to the output signal from said sensing means; and means, connected to receive the first signal from said high-voltage level shifting means, for converting the first signal to a low voltage signal, said low voltage signal being referenced to ground potential.
In accordance with another aspect of the present invention, there is provided an apparatus for generating a low voltage electrical signal proportional to an electrostatic potential on a surface, comprising: sensing means for producing a sensor signal representative of the electrostatic potential on the surface; a high-voltage source adapted to produce a first potential having a first polarity; high-voltage level shifting means, powered by said high-voltage source, for generating a first current signal referenced to ground in response to the sensor signal; a current mirror, connected to the output of said high voltage level shifting means, for receiving the first current signal and producing a second current signal as a function of the first current signal; a second voltage source adapted to produce a second potential at a polarity opposite said first polarity, said second voltage source being adapted to drive said current mirror; and a transimpedance amplifier, connected to the output of said current mirror, for transforming the second current signal produced by said current mirror into a low voltage signal indicative of the electrostatic potential on the surface.